Implantable stimulation devices generate and deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, occipital nerve stimulators to treat migraine headaches, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a microstimulator device of the type disclosed in U.S. Pat. Nos. 7,177,698; 7,120,992; 7,437,193; 7,702,385; 7,920,915; U.S. Patent Application Publications 2006/0276842, published Dec. 7, 2007, and 2004/0015205, published Jan. 22, 2004, all of which are incorporated herein by reference in their entireties. However, the present invention also has applicability in other implantable stimulator devices, such as Spinal Cord Stimulation (SCS) devices, examples of which can be found in U.S. Pat. Nos. 6,553,263 and 6,516,227, which are incorporated herein by reference in its entirety.
A microstimulator device typically comprises a small, generally-cylindrical housing which carries electrodes for producing a desired electric stimulation current. Devices of this type are implanted proximate to the target tissue to allow the stimulation current to stimulate the target tissue to provide therapy for a wide variety of conditions and disorders. A “microstimulator” in the context of this application means an implantable stimulator device in which the body or housing of the device is compact (typically on the order of a few millimeters in diameter by several millimeters to a few centimeters in length) and usually includes or carries stimulating electrodes intended to contact the patient's tissue. Generally, a microstimulator will include at least one anode electrode and at least one cathode electrode, either of which may comprise the housing of the microstimulator if it is conductive. Alternatively, microstimulators can have a plurality of either anodes or cathodes, such as is illustrated in U.S. Pat. No. 7,881,803, which is incorporated herein by reference in its entirety.
FIG. 1 illustrates exemplary circuitry within a microstimulator 10. The illustrated microstimulator comprises a single-anode/multi-cathode design, but could also comprise a single-anode/single-cathode or multi-anode/multi-cathode design. Therapeutic stimulation occurs as follows. The anode electrode 27 sources a current, I, into a resistance 24, R, i.e., the user's tissue. The return path for the current is provided by one or more cathodes 28, which can be selected via cathode switches 30. The magnitude of the current I is specified by a Digital-to-Analog Converter, or DAC 32, whose circuitry and structure is explained in the above-incorporated references. A decoupling capacitor 22, C, is disposed in the current path, usually proximate to the anode electrode 27. As is known, a decoupling capacitor 22 is useful in implantable stimulator devices to assist in charge recovery after the provision of a stimulation pulse, and to provide additional safety by preventing the direct injection of current to the patient's tissue 24.
The microstimulator 10 contains a battery 12 to power its various logic circuits, and to produce the energy necessary to provide the desired stimulation pulses at the electrodes 27, 28. When producing stimulation pulses, it is generally necessary to generate a compliance voltage, V+, from the battery voltage, Vbat. This is because it is generally necessary to tailor the voltage needed to produce the desired therapeutic current, I, and such tailoring is especially necessary when one considers that the resistance 24 of the patient's tissue will be variable.
Generation of the compliance voltage V+ from the battery voltage Vbat is the function of the compliance voltage generation circuitry 18. Compliance voltage generation circuitry 18 generally boosts the battery voltage to a higher compliance voltage V+, and thus comprises a DC-to-DC converter. (The circuitry 18 can also generate a compliance voltage lower than the battery voltage should that be warranted). The compliance voltage generation circuitry 18 uses a coil 15 within the microstimulator. As will be seen below, the coil 15 has other uses in the microstimulator 10. However, as concerns compliance voltage generation, the inductance of the coil 15 is used in conjunction with the V+ generation circuitry 18 (usually including at least one capacitor and at least one diode) to produce a desired compliance voltage, V+. Exemplary V+ generation circuitry employing a coiled inductor to produce the compliance voltage in an implantable medical device is disclosed in U.S. Pat. No. 7,379,775, which is incorporated herein by reference in its entirety.
As just noted, the coil 15 can be used for other purposes within the microstimulator 10. As shown in FIG. 2, the coil 15 can also be used as a means for wirelessly receiving power from an external charger 40, and for wirelessly receiving data from an external programmer 45. These external devices are typically separate from each other, but could be integrated as well. As is well known, the external charger 40 is typically a hand-held device used to recharge the battery 12 within the microstimulator. (In other embodiments, the external charger 40 can also be used to continuously provide energy to an implant otherwise lacking a battery). The external programmer 45 is also typically hand held, and is used by a clinician or patient to send data to the microstimulator 10. For example, by manipulating a user interface (not shown) on the external programmer 45, a clinician can provide a therapy program tailored for a particular patient, which program might specify the amplitude, pulse width, and frequency of the stimulation pulses to be provided to the patient.
These external devices also contain coils 41, 46, which are energized to create magnetic fields, which in turn induce currents in the coil 15 within the microstimulator 10. During charging, energy induced in coil 15 from coil 41 is rectified and passed via charging and battery protection circuitry 14 (FIG. 1) to the battery 12, which allows the battery 12 to be safely charged to a value of about 4.1V for example. During data telemetry, coil 46 in the external programmer 45 is likewise energized, typically using a Frequency Shift Keying (FSK) modulation protocol. Again, the resulting magnetic field induces a current in the coil 15, and the resulting received signal is demodulated at telemetry circuitry 16 to recover the transmitted data. Data telemetry can also occur in the other direction, i.e., from the coil 15 to the coil 46 to allow the microstimulator 10 to report to the external programmer 45 concerning its status, and in this regard the telemetry circuit 16 can comprise both transmitter and receiver circuitry.
From the foregoing, it should be appreciated that the coil 15 in the microstimulator 10 is implicated in compliance voltage generation, battery recharging, and telemetry. The use of one coil 15 to perform different functions in a microstimulator 10 is advantageous: space is limited within the housing of the microstimulator, which tends to limits the number of discrete coils that can be used. Accordingly, it is generally necessary for the microcontroller 20 in the microstimulator 10 to arbitrate or time-multiplex the use of the coil 15 so that the various functions will not be in conflict. For example, during charging, telemetry circuitry 16 and V+ generation circuitry 18 are typically disabled by the microcontroller 20, ensuring that the coil 15 will only be used to assist in recharging the microstimulator's battery 12.
However, compliance voltage generation and data telemetry can generally run concurrently, and it is therefore necessary for the microcontroller 20 to decide which of these two functions can have access to the coil 15 at a given time. To better understand this, it is useful to review how data telemetry operates in the system. Should the external programmer 45 need to communicate with the microstimulator 10, the external programmer 45 will continually broadcast a handshaking message, and wait for an acknowledgment from the microstimulator 10 that it is ready to receive data. The microstimulator 10, in turn, must periodically “listen” for this handshaking message. Such listening occurs only periodically, and only during a listening window 52 of limited duration, D(t), as illustrated in FIG. 3. The telemetry circuitry 16 is enabled, and the coil 15 reserved for telemetry, during the listening window 52. The duration of the listening window 52 may be about 20 milliseconds (ms) or so, and ideally occurs periodically, T(t), every 100 ms or so. However, such periodicity is variable as explained below.
Compliance voltage generation occurs during the provision of therapeutic stimulation to a patient. An exemplary therapy of stimulation 60 is shown in FIG. 3. Essentially, the stimulation 60 can be understood as an alternating sequence of pulses 62 and inter-pulse periods 64. The pulses 62 correspond to points in time in which the desired therapeutic current, I, is provided to patient. Such pulses 62 typically will not exceed a duration D(p) of 1 ms, and may be as low as 10 microseconds in duration.
Two primary events occur during the inter-pulse period 64 after each pulse 62. First, the compliance voltage for the next pulse is generated; this is generally necessary because the issuance of the pulse will have loaded the compliance voltage to below a level suitable for the next pulse. As mentioned earlier, generation of the compliance voltage requires activation of the V+ generation circuitry 18, and access to the coil 15. Second, the decoupling capacitor 22 (C) is discharged during the inter-pulse period 64. As disclosed in the above-incorporated U.S. Pat. No. 7,881,803, this typically occurs by coupling both the anode and the selected cathode(s) to the battery voltage, Vbat, which has the effect of shorting both sides of the decoupling capacitor 22 through the patient's tissue 24. Generally, the duration, D(r), of the inter-pulse period 64 is variable, and depends on the frequency, f(s), of the stimulation pulses chosen as effective for the patient. The inter-pulse period duration generally cannot be less than a certain minimum, which guarantees sufficient time to perform the necessary inter-pulse tasks of compliance voltage generation and output capacitor discharge. The reality of a minimum duration for the inter-pulse period in turns limits the maximum frequency f(s) that can be chosen for the stimulation timing signal 60, but such limit is normally beyond that required for useful therapy and hence does not substantially limit the utility of the microstimulator 10.
As noted earlier, the need to concurrently issue stimulation and to listen for telemetry requires the microcontroller 20 to arbitrate access to the coil 15. How this occurred in the prior art is illustrated in FIG. 3. Illustrated are exemplary ideal timing signals for both telemetry listening (50) and for stimulation (60). As regards telemetry listening, it is seen that the listening windows 52 are ideally set to a duration D(t) of 20 ms, and occur with a periodicity T(t) of 100 ms. The stimulation timing signal 60 in the example has been chosen with a pulse duration D(p) of 1 ms, and a frequency f(s) of 55.555 Hz. Working the math, this equates to an inter-pulse period duration D(r) of 17 ms, for a total stimulation period T(s) of 18 ms.
Assuming these exemplary values, arbitration logic 21 within the microcontroller 20 will cause both the telemetry listening timing signal (50) and the stimulation timing signal (60) to deviate from ideal values. This is because the arbitration logic 21 treats each stimulation cycle 65 (comprising a pulse 62 and an inter-pulse period 64) and each listening window 52 as blocks that cannot overlap in time. Therefore, the microcontroller 20, when arbitrating, will not grant priority to a listening window 52 until the currently-pending stimulation cycle 65 has completed. For example, in FIG. 3, the ideal timing of listening window 52a overlaps with the finishing of stimulation cycle 65a. Therefore, and as shown in the non-ideal telemetry timing signal 50′ resulting from the arbitration, listening window 52a′ is made to wait until the close of stimulation cycle 65a, i.e., until the inter-pulse period 64 within that cycle has completed. Once the listening window 52a′ has issued, it will need unencumbered access to the coil 15. Therefore, the next stimulation cycle 65b′ cannot start until the end of the listening window 52a′, as shown in non-ideal stimulation timing signal 60′.
Because of the arbitration scheme used to mitigate conflicts regarding coil 15 access, a non-ideal telemetry listening timing signal 50′ and a non-ideal stimulation timing signal 60′ result. The non-ideal telemetry listening timing signal 50′ results in a slightly longer period (e.g., T(t)=110 ms) between listening windows 52′ when compared to what was otherwise desired as ideal (100 ms). However, such a minor increase in this period T(t) is not significant or problematic.
By contrast, the resulting non-ideal stimulation timing signal 60′ is potentially problematic. As can be seen in FIG. 3, the result of the arbitration scheme results in prolonged gaps 70a, 70b, etc. in the stimulation pulses 62. These gaps 70a are significantly longer (37 ms) than what was otherwise deemed as ideal therapy for the patient (17 ms), and occur with significant frequency (e.g., every sixth pulse in the example of FIG. 3). Such gross deviations from the ideal may be perceptible by the patient, and hence are greatly disfavored.
Accordingly, the implantable stimulator art, and particularly the microstimulator art, would benefit from an improved technique to allow concurrent stimulation and telemetry listening that does not cause large deviations of the stimulation pulses from their ideal timings.